About this phrase

Ar is the simple preposition meaning on.

Siúl is the verbal noun of the regular verb siúil, meaning walk.

Thus, ar siúl: going on, in progress ⁊rl…

Notes

Not to be confused with the Ulster phrase ar shiúl, meaning go away / gone away:

• Tá sí ar shiúl (she's after leaving).
• Bí ar shiúl! (go away!).
• ⁊rl…

Prepositional pronouns formed from the preposition ar

• orm: on me.
• ort: on you.
• air: on him; on it (masculine).
• uirthi: on her; on it (feminine).
• orainn: on us.
• oraibh: on ye (plural).
• orthu: on them.

The same prepositional pronouns' emphatic forms

• orm-sa: on me.
• ort-sa: on you.
• air-sean: on him; on it (masculine).
• uirthi-se: on her; on it (feminine).
• orainn-ne: on us.
• oraibh-se: on ye (plural).
• orthu-san: on them.